Auscultation of the Heart

The most basic method of assessing cardiac health is thoracic auscultation. For this procedure, the clinician places a stethoscope against the chest wall in the axillary region and then assesses heart rate, rhythm, and strength and listens for any abnormal sounds. The axillary region, the ventral third of the chest between the second rib and the fourth or fifth rib, has relatively little fleece or hair cover, allowing good contact between the bell of the stethoscope and the skin. The clinician should listen to the heart from both sides of the chest at two or three intercostal spaces on each side. Two important considerations in this examination are (1) the need to push the bell under the elbow for assessing the cranial aspects of the heart and (2) the risk of too-ventral placement of the bell, overlying the sternum rather than the thorax, thereby preventing effective auscultation.

Normal heart rate varies with the age of the sheep or goat. Neonatal lambs and kids frequently exhibit rates of 120 to 140 beats/minute, whereas adults of both species often have rates between 66 and 80 beats/minute. Juveniles typically attain an adult rate by 3 months of age. Rates can be increased in stressed or excited animals; such animals should be given time to acclimate to restraint before the clinician assesses their heart rate. Other reasons for tachycardia include anxiety, hypovolemia, venous pooling of blood, arterial hypotension, tachyarrhythmia, and poor cardiac function. Causes of bradycardia include lesions affecting the vagus nerve, bradyarrhythmia, and late-stage shock.

Normal heart rhythm is regular. The most common rhythm anomaly is sinus arrhythmia, in which the heart rate speeds with inspiration and slows with expiration. Reports of other dysrhythmias in sheep and goats are uncommon, but a reasonable presumption is that atrial and ventricular fibrillation, atrioventricular block of variable degree, premature and escape beats, and pathologic tachyarrhythmias occur under conditions similar to those described in other species.

Assessment of the strength of cardiac contractions often is subjective. Sounds are louder on the left side than the right side, and amplitude varies inversely with the body condition of the animal. Both S₁ (closure of the atrioventricular valves or onset of ventricular systole) and S₂ (closure of the semilunar valves or onset of ventricular diastole) should be audible.¹

Peripheral Pulses

Assessment of peripheral pulse strength and synchronicity with cardiac contractions is the simplest way of evaluating the effectiveness of cardiac output. Peripheral arteries can be difficult to find in sheep and goats, especially adult animals. The largest of these vessels are the femoral artery (medial thigh) and the brachial artery (proximal medial foreleg). The facial artery (ventrolateral mandible) and the carotid artery (ventrolateral neck) also can be used for pulse assessment in some animals. Weak or absent pulses are consistent with hypotension and poor cardiac output. Exuberant pulses are consistent with hyperdynamic shock or regurgitation of blood from the aorta into the heart (aortic valve insufficiency) or lung (patent ductus arteriosus with left-to-right shunting). Pulses usually are assessed manually because of the difficulty of placing any sort of manometric device on a conscious sheep or goat.¹

Venous Filling, Pulses, and Pressures

Monitoring jugular vein filling and pulses allows the operator to assess right heart function and blood volume. Sheep and goats with hypovolemia may have small jugular veins that are not visible or palpable even after manual occlusion for several minutes. By contrast, sheep and goats with right heart failure or restrictive pericardial disease may have large jugular veins that are visible or palpable without being occluded and have positive pressures. Pulses in the jugular vein result from backflow of blood during right atrial or ventricular systole. No valve is present to prevent regurgitation during right atrial systole; “weak” pulses that disappear when the head is elevated or do not extend above the level of the heart base are common and nonpathologic. Pulses that extend further up the neck even when the head is elevated most commonly are the result of tricuspid valve insufficiency. Such pulses coincide with right ventricular systole and are caused by regurgitation of ventricular blood through the incompetent valve. Tricuspid insufficiency can occur with right heart failure (and jugular distention) or as a separate entity.

Monitoring venous pressures requires a manometer. The most common form of monitoring involves inserting a fluid-filled line into the jugular vein. The line is attached to a pressure transducer and measuring instrument. Many electrocardiographs also have the capability of measuring pressures. The venous line may be left in the jugular vein or advanced into the central veins and heart. Pressures for the jugular and central veins usually range from negative to as high as 5 cm H₂O. Positive pressures are the result of hypervolemia (caused by excess fluid administration or renal dysfunction), restrictive pericardial disease, and cardiac dysfunction. If venous hypertension becomes severe, especially over a long period, edema develops.¹

Mucous Membrane Assessment

Mucous membranes can be assessed for color, appearance of vessels, hydration, and capillary refill time. The most commonly evaluated mucosae are the buccal, conjunctival, scleral, and vaginal mucous membranes.

Normal mucous membranes are pale pink to pale red, although the high frequency of dark-pigmented membranes in some breeds of sheep and goats sometimes makes this assessment difficult. Overly pale membranes can be attributed to anemia or hypoperfusion; of note, however, ruminant membranes tend to be paler than those in many monogastric species because of their smaller erythrocytes and keratinized membranes. Anemic ruminant membranes often are white, rather than pale pink, and scleral vessels become very small. A semiquantitative color comparison system (FAMACHA) for estimating anemia is available (see Chapters 6 and 16). In some animals, anemia can be differentiated from hypoperfusion by observing capillary refill time after slight digital pressure is applied to the buccal or vaginal membranes. In normal animals, color returns in 1.5 to 2 seconds. A shorter refill time is seen in hyperdynamic shock (which often is accompanied by a reddening of the membranes), and a longer time is seen in hypoperfusion.

Change of the normal membrane color toward a purple or blue hue is indicative of cyanosis. Cyanosis results from poorly oxygenated hemoglobin and can be seen with poor central oxygenation (right-to-left cardiac shunting, pulmonary disease), nonfunctional hemoglobin (methemoglobinemia or sulfhemoglobinemia), and local vascular stasis (hypodynamic shock, poor cardiac output, hypothermia). With poor central oxygenation, purple-blue discoloration of all mucous membranes and possibly of nonpigmented skin is characteristic, whereas with vascular stasis, only certain areas such as the gingival margins may be affected. Scleral vessels often become engorged, tortuous, and purple during vascular stasis. Approximately one third to one half of blood hemoglobin must be deoxygenated for membranes to become cyanotic; therefore cyanosis usually occurs only when blood oxygen partial pressures (local or central) are already very low.

Accumulation of bilirubin leads to yellow discoloration of the mucous membranes—a condition termed jaundice or icterus. Icterus can develop as a result of intravascular or extravascular hemolysis, decreased hepatic uptake of bilirubin, and decreased biliary excretion; the hemolytic causes are most common in sheep and goats.

Hydration can be assessed by observation of the moistness or tackiness of the mucous membranes. This is a subjective determination that is improved by practice on normal animals. A loss of body water suggests that fluids should be part of the treatment protocol. However, dehydration becomes clinically apparent only when body fluid loss exceeds 5% of total body weight (see Chapters 3 and 16).

Blood Gas Analysis

Analysis of blood gases can provide valuable information about animals with hypoperfusion. Metabolic acidosis in sheep and goats without diarrhea, ketonemia, or grain overload often results from lactic acid production by underperfused tissues. Venous blood for analysis can be obtained from any accessible peripheral vein; the blood specimen should be collected anaerobically and stored in a heparinized container. A determination of whether underperfusion is attributable to inadequate blood oxygen content (pulmonary gas exchange) or inadequate tissue blood flow (blood volume and pressure) requires arterial blood gas analysis. Arterial blood most commonly is collected from the brachial, femoral, or medial saphenous artery, but these arteries are poorly accessible in vigorous animals. Clinicians must avoid unnecessary stress when restraining sick animals for blood collection. The auricular arteries and peripheral limb arteries can be used in anesthetized patients. Inadequate arterial blood oxygen content suggests right-to-left cardiac shunting (as in right-to-left patent ductus arteriosus or septal defect with or without abnormalities of the great vessels) or pulmonary disease. Differentiation among the causes of inadequate arterial blood oxygen content requires an extensive cardiopulmonary examination.¹

Electrocardiogram

Electrocardiographic evaluation is most useful for sheep and goats with cardiac dysrhythmias. The most common technique uses the base-apex lead: The positive electrode (LA) is placed over the cardiac apex in the left fifth intercostal space at the level of the elbow, the negative electrode (RA) is placed in the right jugular furrow at the height of the base of the heart, and the ground (LL) is placed on the dorsal spine or another site distant from the heart. Topical application of alcohol improves skin contact, and clipping of fleece may be necessary if the complexes are small (Figure 15-1). Presence of behaviors such as panting and muscle tremors often leads to baseline interference in adult sheep.

The electrocardiogram (ECG) should reveal a distinct P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization). The R component (negative deflection after a positive deflection) of the QRS complex usually is the most prominent, and the Q component (negative deflection before the first positive deflection) usually is absent. The T wave can be either positive or negative and may vary on a single strip.

The ECG should be evaluated for regular appearance of P waves and QRS complexes; regular P-P, R-R, and P-R intervals; presence of P waves and QRS complexes that are identical in appearance; and presence of T waves of normal amplitude. The Q-T interval varies inversely with heart rate. An absence of P waves indicates atrial fibrillation or ascension of a ventricular or supraventricular pacemaker. The absence of QRS complexes indicates atrioventricular block.¹

Echocardiography

Echocardiography is a diagnostic ultrasound imaging modality that is safe, noninvasive, and convenient to perform in a standing animal; however, it is seldom used in small ruminants because of the expense. Nevertheless, it is extremely useful in sheep and goats to confirm presence of intracardiac and pericardial diseases, including valvular endocarditis, pericarditis and pericardial effusions, cardiomyopathy, congestive heart failure, and congenital heart defects. Echocardiography can be used to assess heart chamber size, valve motion, wall thickness of various structural components, blood flow, and intracardiac hemodynamics.

Echocardiography is performed in three basic modes:

With these ultrasound techniques, pressure gradients can be estimated within the heart and great vessels such as the pulmonary artery and aorta. The two main Doppler modes are pulsed wave and continuous wave. Another modality in common use, Color flow Doppler, converts the Doppler signals to an arbitrarily chosen color scale for semiquantitative evaluation of the direction, velocity, and turbulence of blood. The use of color flow Doppler is restricted mainly to specialty practices and referral institutions, because significant expertise and experience are required to interpret the findings, and the equipment is cost-prohibitive for many smaller practices.

Echocardiography can be performed in a standing or laterally recumbent small ruminant. The best location for placement of the transducer is over the right third intercostal space at the level of the elbow; a high-frequency transducer that fits well in the intercostal space should be selected. The same spot on the left side can be used if the entire heart cannot be visualized from the right side. The examiner may choose to clip the fleece of a sheep or the hair of a goat to improve image resolution before applying the coupling gel.

Performing echocardiography involves systematic interrogation (examination) of cardiac structures to determine chamber size, myocardial function, valve appearance and motion, and aorta and pulmonary artery blood flow and assessment for the presence of abnormalities within or around the heart. All cardiac structures should be imaged on both long- and short-axis views.¹

Other Imaging Modalities

Thoracic radiographs are used diagnostically by many private practitioners with access to portable radiographic equipment or a stationary small animal x-ray machine. Most sheep and goats can be examined using the same radiographic techniques as for large dogs. Radiographic studies are most helpful if the practitioner suspects that heart failure, valvular lesions, or abnormal extracardiac or intracardiac communication may be causing chamber or great vessel enlargement or dilatation. Some pulmonary disorders also can be visualized.

Lateral thoracic radiographs are easiest to obtain in sheep and goats, and the imaging may be performed with use of physical restraint only. Ventrodorsal views may require use of sedation and are of limited usefulness in deep-chested small ruminants. Dirt and foreign bodies in the fleece of sheep can create artifacts on the radiographic film, so the fiber should be examined before the radiograph is taken.¹

Cross-sectional imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI) have seen limited use in sheep and goats because of expense and availability, but their use will increase as more practices invest in this type of equipment. Both of these modalities are useful for identifying subtle morphologic defects that might be obscured by overlying tissue on conventional radiographic examination, such as vascular anomalies or other cardiac malformations. Sheep and goats are relatively easy to image, because they fit on equipment designed for people or larger small animals.

Heartwater Disease (Cowdriosis)

Heartwater disease, or cowdriosis, is an acute, tick-borne septicemic disease caused by the rickettsial organism Ehrlichia (formerly Cowdria) ruminantium. All ruminants are susceptible, particularly Merino sheep and Angora goats. Birds, tortoises, and various mammals also can serve as hosts. The disease is not contagious and is transmitted by ticks of the genus Amblyomma, particularly Amblyomma hebraeum (the bont tick) and Amblyomma variegatum. The rickettsial organism is found in the intestinal epithelial cells of its vector. The host tick requires three blood meals to complete its life cycle; only the third host must be a large mammal. Infected ewes and does develop ovarian infections and pass E. ruminantium vertically to their offspring. Transcolostral transmission has been described as well.

The disease is largely confined to areas in which ticks of the genus Amblyomma are prevalent, including sub-Saharan Africa, Madagascar, some islands in the Indian Ocean and the Caribbean Sea, and Europe. Reports have described the recovery of infected ticks from imported tortoises in Florida, and uninfected vector ticks from people and birds in the United States.^4–6 Some North American Amblyomma ticks have been shown to be competent vectors, but the disease has never established itself in the United States. Heartwater disease severely impairs ruminant health and husbandry in disease-endemic areas.